So-called “impregnated” drag bits are used conventionally for drilling hard and/or abrasive rock formations, such as sandstones. Such conventional impregnated drill bits typically employ a cutting face having blades or inserts comprising superabrasive cutting particles, such as natural or synthetic diamond grit, dispersed within a metal or metal alloy matrix material. As such a bit drills, the matrix material wears away, exposed cutting particles are lost as the surrounding matrix material to which the particles are mechanically and metallurgically bonded is removed, and new cutting particles previously buried within the matrix material become exposed. These diamond particles may be cast integrally with the body of the bit, as in a low-pressure infiltration process to form blades comprising the diamond particles and matrix material, or inserts comprising the diamond particles and matrix material may be preformed separately from the bit body, such as in a hot isostatic press (HIP) sintering process, and the inserts may be attached subsequently to the bit body by brazing. In other processes, such preformed inserts may be placed within a mold in which the bit body is cast using an infiltration process. In such a process, the inserts become bonded to the bit body as the bit body is formed over and around the inserts.
Conventional impregnated bits generally exhibit a poor hydraulics design by employing what is referred to in the industry as a “crow's foot” to distribute drilling fluid across the bit face and providing only minimal flow area. Further, conventional impregnated bits do not drill effectively when the bit encounters softer and less abrasive layers of rock, such as shales. When drilling through shale, or other soft formations, with a conventional impregnated drag bit, the cutting structure tends to quickly clog or “ball up” with formation material, making the drill bit ineffective. The softer formations can also plug up fluid courses formed in the drill bit, causing heat buildup and premature wear of the bit. Therefore, when shale-type formations are encountered, a more aggressive bit is desired to achieve a higher rate of penetration (ROP). It follows, therefore, that selection of a bit for use in a particular drilling operation becomes more complicated when it is expected that formations of more than one type will be encountered during the drilling operation.